1. Field of the Invention
The present invention relates to a coating film forming method and a coating apparatus each for forming a coating film, for example, a resist film on the front face of a substrate such as a semiconductor wafer or the like.
2. Description of the Related Art
For instance, in a photolithography process of a semiconductor device, a photoresist is applied to a semiconductor wafer, a circuit pattern is transferred to the photoresist using photolithography technology, and the circuit pattern is developed, thereby forming a circuit. In the above process, included is a coating film forming process for applying a resist solution onto the front face of the semiconductor wafer.
In the forming method of a resist film in this case, as shown in FIG. 13, the semiconductor wafer W is mounted on a spin chuck (illustration thereof is omitted) in a processing case and the wafer W is rotated by a drive motor 201. Then, the resist solution is dripped from a resist solution supply nozzle 203 onto the center of the wafer W by a pump 202 such as a bellows pump or the like. Thereby, as shown by many arrows in FIG. 13, the resist solution R is diffused from the center of the wafer W toward the surrounding portion by centrifugal force. Thereafter, the rotation of the wafer W is kept to adjust the thickness of the resist solution R diffused on the wafer W to form a resist film with a uniform film thickness on the wafer W and to blow off an excess resist solution.
As shown by many arrows in FIG. 13, when the resist solution R diffuses from the center of the wafer W toward the surrounding portion by centrifugal force, the circular outline of the outer periphery of the resist solution R spreads while the diameter thereof is increasing by centrifugal force and finally reaches the outer edge of the wafer W keeping its circular form. Thereby, the resist solution R is applied on the entire face of the wafer W in a uniform thickness.
However, when the resist solution R diffuses from the center of the wafer W toward the surrounding portion, as shown in FIG. 14, as for the outline in circular form of the resist solution R, it occurs that the circular form can not be kept and portions thereof which break the circular outline to protrude outward appear in various positions resulting in a waveform outline. This outline in waveform is called xe2x80x9cirregular waveformxe2x80x9d which means that the resist solution R can not spread uniformly on the wafer W. If its degree is serious, xe2x80x9cuncoatingxe2x80x9d of the resist solution occurs and thus there is a danger that a resist film can not be formed on the entire surface of the wafer W uniformly in film thickness.
As the reason why the scratchpad S occurs, it is suspected that the centrifugal force acts greatly with the circular outline of the resist solution R spreading outward, thereby increasing the spreading speec of the outline, and as a result, portions breaking the circular outline appear in various positions.
By the way, in recent years, the amount of the resist solution dripped is reduced from the viewpoint of reducing the amount of the resist consumed because of a reduction in fabrication cost and the like. Therefore, a solvent such as a thinner or the like is applied before applying the resist solution to improve wettability of the resist solution, thereby preventing an irregular waveform. However, when the amount of the resist is little, an irregular waveform is prone to occur, and also when the wettability is poor, an irregular waveform is easily to occur.
To avoid xe2x80x9cuncoatingxe2x80x9d by preventing this kind of irregular waveform, conventionally it is necessary to apply a resist solution in amount slightly more excessive than originally needed, on the contrary to the demand to reduce as much as possible the consumption of resist in the result. SUMMARY OF THE INVENTION
An object of the present invention is to provide a coating film forming method and a forming apparatus each for preventing occurrence of an irregular waveform or decreasing the degree thereof to thereby avoid uncoating of the coating solution on a substrate and for reducing the amount of the coating solution used.
To solve the above disadvantage, according to a fist aspect of the present invention, a coating film forming method for supplying a coating solution from a coating solution supply nozzle onto the front face of a substrate housed in a processing case to form a coating film comprises the steps of: mounting the substrate on a mounting table and rotating the substrate; discharging the coating solution from the coating solution supply nozzle onto almost the center of the substrate and diffusing the coating solution from almost the center of the rotated substrate toward the outer edge; detecting a spreading state of an outline of the outer periphery of the diffusing coating solution; and controlling at least one of the rotation speed of the substrate, the amount of the coating solution discharged from the coating solution supply nozzle, and the speed of the coating solution discharged from the coating solution supply nozzle based on the detected state.
According to a second aspect of the present invention, a coating apparatus comprises: means for rotating a substrate; means for discharging a coating solution onto almost the center of the rotated substrate; detecting means for detecting a spreading state of an outline of the outer periphery of the coating solution when the discharged solution is diffused from almost the center of the rotated substrate toward the outer edge; and means for controlling at least one of the rotation speed of the substrate, the amount of coating solution discharged, and the speed of the coating solution discharged based on the detected state.
In the study results of the inventor, it has been found that an irregular waveform greatly depends on the spreading speed of the coating solution, that is, an irregular waveform occurs when the spreading speed of the coating solution becomes not less than a predetermined speed with an increase in centrifugal force, and thus an irregular waveform can be prevented by properly controlling the spreading speed of the coating solution, and that even if an irregular waveform occurs, the irregular waveform up to a certain degree does not become a cause of uncoating.
Therefore, in the aforesaid present invention, for instance, the spreading state of the outline of the outer periphery of the coating solution diffusing from almost the center of the rotated substrate toward the outer edge is detected, the spreading speed is calculated from the spreading state, and the rotation speed of the substrate and/or the amount or the speed of the coating solution discharged from the coating solution supply nozzle are/is controlled so that the spreading speed becomes not more than a predetermined speed, thereby preventing an irregular waveform. Consequently, uncoating on the substrate can be prevented and the coating solution can be applied uniformly in film thickness. Therefore, it is unnecessary to supply an excessive coating solution unlike the prior art, and the amount of the coating solution used can be reduced further. Moreover, the control as above can prevent occurrence of an irregular waveform even if wettability of the solvent applied before the coating solution is poor, thereby preventing occurrence of uncoating of the coating solution.
Further, in the aforesaid present invention, for instance, the spreading state of the outline of the outer periphery of the coating solution diffusing from almost the center of the rotated substrate toward the outer edge is detected, the spreading state of the outline of the outer periphery of the coating solution is detected when the coating solution is diffused from almost the center of the rotated substrate toward the outer edge, and when an irregular waveform formed by breaking the outline of the outer periphery of the coating solution is detected, the width in the radius direction of the irregular waveform is kept, and the rotation speed of the substrate and/or the amount or the speed of the coating solution discharged from the coating solution supply nozzle are/is controlled so that the above width becomes not more than a predetermined value. Thereby, the irregular waveform can be prevented from becoming so large as to exert influence upon application of the resist solution, so that uncoating on the substrate can be prevented and the coating solution can be applied uniformly in film thickness. Consequently, similarly in this case, it is unnecessary to supply an excessive coating solution unlike the prior art, the amount of the coating solution used can be reduced further, and even if wettability of the solvent applied before the coating solution is poor, occurrence of uncoating of the 5 coating solution can be prevented by the above control.